1. Field of the Invention
This invention relates generally to circuitry for providing a preselected driving point immittance and, more particularly, to frequency-sensitive driving point immittance circuitry derivable from a wideband transfer function.
2. Description of the Prior Art
In many telecommunications applications, it is necessary to couple a bidirectional two-wire line to a pair of unidirectional two-wire lines oftentimes referred to as a four-wire line. In a telephone system, for example, bidirectional signals carried over a two-wire subscriber loop are split into separate transmit and receive unidirectional signals at a subscriber terminal and a central office. A network generally utilized to effect this coupling function is called a hybrid circuit.
A conventional hybrid circuit comprises a biconjugate four-port network wherein the two unidirectional lines are connected to one set of conjugate ports and the bidirectional line and a balancing network are connected to the other set of conjugated ports. In the telephony example cited above, an impedance mismatch between the subscriber loop and the hybrid balancing network may result in a signal reflection which, in turn, can result in instability of amplifiers appearing in the system or echoes which are subjectively objectionable to the telephone users. Subscriber loops present a wide range of impedances due to numerous cable gauges which typically comprise a loop as well as the varying lengths of each gauge. Moreover, the overall length distribution of loops displays wide variation.
In the prior art, it has often been assumed that the loop impedance is relatively constant so that the hybrid network comprises only resistors. The distortion produced because of imperfect balancing can be ignored for narrowband voice signals used in standard telephony. However, for those applications requiring a high degree of balance, it has been necessary to either construct precision balancing networks which can be adjusted to match the loop impedance or to manufacture a large number of different balancing networks. In these applications, the balancing networks have been complex, expensive, rather large in size and typically require inductive elements.
In modern telephone systems, it is necessary to connect wideband digital data terminals to the various subscriber loops. The problem of matching a loop becomes particularly acute because the loop impedance is frequency sensitive over a broad frequency band. In my U.S. Pat. Nos. 4,174,470 and 4,181,824, issued Nov. 13, 1979 and Jan. 1, 1980, respectively, I disclose an electronic hybrid and an associated balancing network. The network is synthesized using frequency-sensitive RC networks to achieve a better match to a connected loop over a broad frequency range. However, the synthesis technique is of the driving point type, as contrasted to transfer immittance type, so more control parameters are needed to achieve a desired balancing network characteristic. Moreover, the individual capacitor components may be quite large and therefore the networks may not be fabricated with integrated circuits.
The disclosure of U.S. Pat. No. 3,919,502, issued to Daryanani on Nov. 11, 1975, presents a technique for realizing a driving point impedance utilizing transfer immittance synthesis. Embodiments of this technique include balancing networks in voice frequency telephone applications. Complex impedance variations can be matched more easily with a transfer impedance network, as compared to a strictly two-terminal network, because the control parameters are basically independent. Although the circuit configurations disclosed by Daryanani are useful in the voice frequency range, the configuration possess certain limitations precluding their application to wideband services. The configurations exhibit narrowband characteristics because operational amplifier circuitry with negative feedback is employed. Moreover, the high-gain negative feedback yields circuit configurations which are not inherently stable.